A) Field of the Invention
The present invention relates to a ZnO (zinc oxide) based compound semiconductor light emitting device and its manufacture method.
B) Description of the Related Art
A semiconductor light emitting device having an active layer made of ZnO based compound semiconductor is known as disclosed in JP-A-2002-111059 and JP-A-2004-342732. ZnO based compound semiconductor includes not only ZnO but also mixed crystal such as MgZnO (magnesium zinc oxide) and CdZnO (cadmium zinc oxide) having ZnO as host crystal.
FIGS. 5A and 5B are schematic cross sectional views of semiconductor light emitting devices having an active layer made of ZnO based compound semiconductor.
With reference to FIG. 5A, a manufacture method for a semiconductor light emitting device having an active layer made of ZnO based compound semiconductor will be described.
An n-type ZnO buffer layer 52 having a thickness of 10 to 1000 nm is formed on an n-type ZnO substrate 51 at a temperature of 300 to 500° C. Next, an n-type ZnO layer 53 doped with Ga and having a thickness of 1 μm or thicker is formed on the n-type ZnO buffer layer 52. The thickness of 1 μm or thicker mitigates the influence of defects invading from an interface between the n-type ZnO substrate 51 and n-type ZnO buffer layer 52 and sufficiently retains the functions of upper layers over the n-type ZnO layer 53.
An n-type MgZnO layer 54 doped with Ga is formed on the n-type ZnO layer 53. The n-type MgZnO layer 54 has functions of an n-type carrier injection layer and of a carrier confinement layer.
Next, an active layer 55 is formed on the n-type MgZnO layer 54. The active layer 55 has, for example, a double hetero (DH) structure or a quantum well (QW) structure.
In the case of the DH structure, the active layer 55 is made of an undoped ZnO layer, CdZnO layer, ZnOS layer, ZnOSe layer or ZnOTe layer. In the case of the QW structure, the active layer 55 has a lamination structure of, e.g., a thin film MgZnO/ZnO (or CdZnO, or ZnOS, or ZnOSe or ZnOTe)/MgZnO.
A p-type MgZnO layer 56 doped with N is formed on the active layer 55. The p-type MgZnO layer 56 has a function of a p-type carrier injection layer. The p-type MgZnO layer 56 has a low carrier concentration and a low carrier mobility and therefore has a high resistivity. Therefore, in order to form an ohmic electrode, a p-type ZnO layer 57 doped with N is formed on the p-type MgZnO layer 56.
After the p-type ZnO layer 57 is formed, a p-side ohmic electrode 58 is formed. The p-side ohmic electrode 58 is an electrode formed on a partial surface area of the p-type ZnO layer 57, and has a circular form, for example.
An n-side ohmic electrode 50 is formed on the surface of the n-type ZnO substrate 51 opposite to the n-type ZnO buffer layer 52. For example, the n-side ohmic electrode 50 is made of Al having a thickness of 100 nm.
In the semiconductor light emitting device shown in FIG. 5A, light is output from the p-type side having a high resistivity (low carrier mobility). This is why the p-side ohmic electrode 58 is formed on a partial surface area.
Due to a large effective mass of holes, the mobility of holes is, for example, as small as several cm2/Vs. As a result, the p-type ZnO layer 57 exhibits a high resistivity. Each layer of the semiconductor light emitting device shown in FIG. 5A has a very small size in a thickness direction as compared to that in an in-plane direction. Therefore, as current is made to flow in the semiconductor light emitting device having the structure shown in FIG. 5A, current flows mainly in the thickness direction so that the current is likely to be injected only in the region just under the p-side ohmic electrode 58 on the partial surface area, and hardly diffuse in the in-plane direction of each layer. Therefore, emission of the active layer 55 occurs only in the region just under the p-side ohmic electrode 58. Most of radiated light may be shielded by the electrode and will not be output to the exterior in some cases.
FIG. 5B shows a modification of the semiconductor light emitting device shown in FIG. 5A. This device of FIG. 5B differs from the semiconductor light emitting device shown in FIG. 5A in that a transparent electrode 59 having a thickness of 15 nm and made of, e.g., Ni, is formed on a p-type ZnO layer 57, and a p-side bonding electrode 60 having a thickness of 100 nm and made of, e.g., Au, is formed on the transparent electrode 59.
This transparent electrode 59 can solve the problem that most of radiated light may be shielded by the electrode and will not be output to the exterior. However, in the manufacture of a semiconductor light emitting device, an additional process is required for forming the transparent electrode 59. Another problem is that radiated light is absorbed in the transparent electrode and the amount light output is reduced.
It can be considered that a p-type ZnO substrate is prepared and a p-type ZnO layer and other layers are formed from the p-type side to manufacture a semiconductor light emitting device having a light output plane on the n-type ZnO layer side. In this case, it is difficult to form a device having good crystallinity. In forming a p-type ZnO layer, it is necessary to set an impurity doping amount larger than when the n-type ZnO layer is formed, and crystallinity is degraded as the doping amount is increased. This is because a large amount of impurities doped in the p-type ZnO layer adversely affects each layer to be formed thereafter.
It is also difficult to form a p-type ZnO substrate itself. A large amount of impurities is required to form a p-type ZnO substrate. However, the substrate is required to be formed under a nonequilibrium state because the solubility of impurities is small in an equilibrium state. However, a large sized, thick substrate is produced industrially through growth in an equilibrium state.